The present invention relates to a process carried out in conjunction with the phosphating of a metal surface which consists, at least in part, of iron or steel and is dipped into, or flooded with, a phosphating solution. The solution contains layer forming cations and nitrate or equivalent accelerators. The iron content of the solution is limited by the precipitation of iron phosphate in that a partial volume of the phosphating solution is intermittently withdrawn from the bath tank and is contacted with oxygen or an oxygen-containing gas in a separate aerator. Iron phosphate sludge is removed from the resulting solution and the solution is then returned to the bath tank.
In the formation of a phosphate coating on a metal surface by a treatment with, e.g., a zinc phosphate solution, it is common practice to add one or more oxidizers to the phosphating solution to accelerate the formation of the layer. In the formation of phosphate coatings on iron or steel surfaces, special problems arise because iron is dissolved and initially forms iron(II) ions in solution.
In one category of phosphating processes, the phosphating solutions used contain oxidizers for transforming iron(II) to iron(III) so that an insoluble iron phosphate is formed. As additional surface area is treated, sludge forms in considerable amounts and must be removed by physical separation. Difficulties are involved in the removal of the sludge in some processes so that it is more desirable to control the phosphating process so as to preclude, or minimize, the formation of sludge.
In the phosphating processes of that category, which are said to be carried out "on the iron side", the phosphating solution contains chemicals which do not transform dissolved iron to a trivalent state. This is effected by the use of e.g., nitrate, or a similarly weak oxidizer as accelerator.
Whereas in the phosphating processes carried out "on the iron side" the formation of sludge is substantially suppressed, such processes involve other disadvantages. For instance, because a strong oxidizer is absent, the phosphate layer forms slowly. Since the solution is enriched with iron(II) phosphate, the content of iron phosphate in the resulting phosphate layer may become undesirably high and the formed phosphate layers tend to be coarse-grained.
Numerous attempts have been made to eliminate these various problems. For instance, in the process in accordance with GB-A-996,418, urea is added to the phosphating bath so that the phosphating can be carried out at a higher temperature without the risk of the formation of much more sludge. While that process results in a faster formation of the phosphate layer, the other mentioned disadvantages remain. An additional disadvantage is the phosphating solution must be heated and this otherwise unrequired heating consumes additional energy.
In a different bath of the type which is not operated "on the iron side", it has been attempted to solve the sludge problem by increasing the solids content of the sludge so that less sludge is formed (GB-A-1,555,529). Whereas a compact sludge is obtained and the interval between the times at which the treating apparatus must be desludged is increased, it has been found that it may be more difficult to remove compact sludge rather than light-weight, bulky sludge.
It is known from EP-A-45 110 to form phosphate layers on iron or steel surfaces, which are dipped into, or flooded with, solutions which have a defined composition and in which an iron(II) content of 0.05 to 1% by weight is adjusted by the addition of a suitable amount of ClO.sub.3 or a similarly acting accelerator, which oxidizes iron(II) to iron(III).
In another process of phosphating a metal surface which consists at least in part of iron or steel and is dipped into or flooded with phosphating solutions which contain layer-forming cations, particularly zinc cations, and nitrate or equivalent accelerators, a partial volume of the phosphating solution is withdrawn from the bath tank, oxidizers are added to that partial volume in a separate device to effect a precipitation of iron phosphate, and iron phosphate sludge is removed from the resulting solution. The solution is then returned to the tank. Iron phosphate is mainly precipitated by an addition of chlorate and/or hydrogen peroxide. The amount of precipitated iron phosphate is desirably controlled so that the iron content of the phosphating solution in the bath tank is not in excess of the content of the cation which determines the type of layer formed (DE-A-33 45 498).
Whereas the two last mentioned processes are satisfactory as regards the control of the iron(II) content of the phosphating solution, handling of the phosphate sludge which is formed is not solved in a satisfactory manner.
It is an object of the present invention to provide a process which is carried out in conjunction with a metal surface consisting at least in part of iron or steel and which is free of the known disadvantages, particularly those mentioned hereinbefore, does not require additional chemicals, can be carried out in a simple manner without adversely affecting the quality of the layer, does not require a substantial apparatus expenditure and permits a simple handling of the sludge.